Modern governments work from a grid of planned announcements which are designed to shape the daily political agenda in their favour. Sometimes it works. Sometimes it doesn't. Ever since the budget, the coalition's grid has been a shambles. On Friday, the home secretary tried to head off the bad press about George Osborne's budget by unexpectedly trying to change the subject to alcohol abuse. Yesterday it was the Cabinet Office minister Francis Maude's turn – in the prime minister's ill-advised absence – to fight the fires lit by revelations about cash for access to Conservative ministers. In the event, he only poured petrol on the flames. It is all, for the moment, proving a bit of a nightmare for the coalition government gridmasters. But if the lord of political misrule is working behind the scenes to a secret grid for retoxifying the Tory brand, then at least that one is all going brilliantly.

Mr Maude's statement came against the backdrop of David Cameron's realisation that the former Tory treasurer Peter Cruddas has dropped him into much deeper political ordure than he first appreciated. The Cruddas claims, too easily dismissed by the government as mere bluster, are extremely serious. They involve not just suggestions of cash for access to ministers but suggestions of cash for influence – meet David Cameron and ask him any question – and even cash for policy change. Ed Miliband may not have been accurate to imply yesterday that in the Cameron-Cruddas Tory party it is possible for rich donors to get the budget changed to suit their interests. But polling during the MPs' expenses scandal made it clear that the public is only too ready to believe this sort of corruption is commonplace. This is a damaging and perilous episode for Mr Cameron's credibility.

The Tories decided that attack is the best form of defence. Forced to deal with the Cruddas fallout in an angry House of Commons, Mr Maude showed only bad-tempered contrition. Instead of humility and an acceptance that all the parties, including his own, had got it wrong over donations, Mr Maude was at his most partisan, laying into Labour for its refusal to tackle the trade union connection.

Many of Mr Maude's charges were right. Labour's defence of the opt-out system for trade union members' funding is anachronistic and self-interested. Labour was never serious enough about political funding reform while it was in government. And Labour was just as guilty of using internal investigations to head off embarrassing scandals of its own as the Tories now are in the Cruddas case. But this yah-boo approach does nothing for the public interest. The charge and counter-charge, and the bad-tempered tit for tat between the parties, will only confirm the public's view that the parties are all in this together, and that they are all a bunch of scoundrels.

That belief is far closer to the truth than is good for the public interest. Mr Cameron's sudden volte-face on publication of the details of some of his dinners with donors was good as far as it goes. But the deeper truth is that both the main parties still conspire to block necessary reform on political donations. The Tories want a system based on the large private donations which suit them and which do down Labour. Labour wants the union financial link that works for them but to clamp down on donations that hurt the Tories.

It was only last November that Sir Christopher Kelly's committee on standards in public life produced a balanced and sensible plan to resolve the stalemate between the parties, with a strict cap on donations, an opt-in system for union funding, a limit on spending outside elections and a modest increase in public funding for the parties. It is not ideal, and the public would still be suspicious. But it would knock the sleazy worlds of Mr Cruddas and the union barons on the head and would make corruption far more difficult. Kelly showed the way forward, but everything that has happened since then, including yesterday's Commons exchanges, show that the parties are still determined to block fair reforms.